This proposal is intended at addressing a major question with regard to selective ototoxic damage to sensory hair cells of the fish ear caused by one aminoglycoside antibiotic, gentamicin sulfate. Recently we demonstrated that only striolar hair cells in the utricular and lagenar otolithic endorgans of fish ear are susceptible to gentamicin sulfate assault. The same type of selective damage to the cochlear outer hair cells and type I hair cells of vestibular endorgans have also been reported in mammalian species. These identical damage patterns between fish and mammals justify the fish ear as a valuable model system for the proposed study. Three hypotheses are proposed and will be used to elucidate mechanisms leading to selective damage to sensory hair cells caused by gentamicin. 1) "Receptor Presence " hypothesis predicts differential distribution of gentamicin receptors between the two types of hair cells. It will be tested by localizing the presence of gentamicin receptors, i.e., phosphoinositide (PIP2). Immuno-gold transmission electron immunocytochemical procedures with antisera raised against the receptor will be used to detect the distribution of receptors between striolar and extrastriolar hair cells. 2) "Route Pathways" hypothesis predicts spatial and temporal difference in uptake routes of gentamicin between the two cell types. The hypothesis will be tested by using both autoradiography and immunocytochemistry methods. Tritiated gentamicin will be used to visualize the intracellular presence of gentamicin under light microscopy. Antisera raised against gentamicin will be used in conjunction with immuno-gold to examine intracellular targets of gentamicin under transmission electron microscope. 3) "Lithium blockage" hypothesis predicts that if gentamicin receptors could be blocked then no ototoxic damage should be observed. The hypothesis will be tested by using lithium, a known inhibitor of PIP2, through systemic injection and direct application into otolithic endorgans in conjunction with injections of gentamicin. "Lithium blockage" hypothesis could be proved if no ototoxic damage is detected at striolar hair cells.